1. Field of the Invention
The invention relates to a circuit using Josephson tunnelling devices and more particularly, to a superconductive shift register circuit which requires no sustaining power for operation.
2. Description of the Prior Art
Josephson tunnelling devices are superconductive elements exhibiting a zero voltage current stage in which pair tunnelling exists, and a finite voltage state in which single particle tunnelling exists. The existence of a zero voltage state in a superconductive tunnel junction was first described in July, 1962 by B. D. Josephson. Since that time, these devices have been proposed for applications in memory and logic. For instance, U.S. Pat. No. 3,626,391 describes a superconductive memory using Josephson tunnelling devices in which memory cells comprised of superconducting loops are used. Josephson junctions determine the direction of current flow in the superconducting loops and they are also used for sensing the current in these loops.
U.S. Pat. No. 3,281,609 describes a logic device using Josephson tunnelling junctions in which the magnetic fields applied to the junction cause the junction to switch voltage states, depending upon whether or not the maximum zero voltage current through the junction is exceeded. Externally applied magnetic fields are used to lower the threshold current (maximum zero voltage current) of the tunnel junction so that switching to a finite voltage state occurs.
U.S. Pat. application, filed June 30, 1972, Ser. No. 267,841, now U.S. Pat. No. 3,758,795, describes a superconductive circuit using a Josephson tunnelling device connected to a transmission line having a termination such that reflections do not result when the Josephson tunnelling device switches between two stable voltage states, in accordance with applied input signals.
Josephson tunnelling junctions have been applied to various logic circuits. However, they have not been utilized in a shift register circuit which is capable of providing shifting without the necessity of any standby power to sustain operation. In other words, each of the stages is in the superconductive current state in which there is no voltage regardless of whether the stage is in the so-called "1" or "0" state. The register shifting can be stopped at the completion of any phase time and substained in that condition with no standby power dissipation. To reinitiate shifting, it is only necessary to bring in the next phase clock.
Although applications for Josephson tunnelling junctions are known in the prior art, the prior art does not show how to obtain a reliable shift register circuit, especially one that can be implemented using very few Josephson devices and which requires a minimum of power.
Accordingly, it is a main object of the present invention to provide a shift register circuit utilizing Josephson tunnelling devices in which the power dissipation is minimized.
It is another object of the present invention to provide a shift register circuit using Josephson tunnelling devices in which the operation can be obtained in a two-phase timing sequence.
It is still another object of the invention to provide a shift register circuit using Josephson tunnelling devices which is capable of extremely high speed switching.
It is a further object of the invention to provide shift register circuits using Josephson tunnelling devices in which the various stages have the same current supply source.
It is a still further object of the invention to provide Josephson tunnelling device circuits which can be easily fabricated using conventional planar technology.